I. Field of the Invention
The present invention relates generally to the field of wireless communications. More specifically, the present invention relates to a method for reducing call dropping rates in a wireless communication system having multiple beam communication links.
II. Related Art
There are a variety of wireless communication systems having multiple beam communication links. A satellite-based communication system is one such example. Another example is a cellular communication system. A satellite-based communication system includes one or more satellites to relay communications signals between gateways (also referred to as "communication stations" or "base stations") and user terminals. Gateways provide communication links for connecting a user terminal to other user terminals or users of other communications systems, such as a public telephone switching network. User terminals can be fixed or mobile, such as a mobile telephone, and positioned near a gateway or remotely located.
A satellite can receive signals from and transmit signals to a user terminal provided the user terminal is within the "footprint" of the satellite. The footprint of a satellite is the geographic region on the surface of the earth covered by the satellite communication system. In some satellite systems, a satellite's footprint is geographically divided into "beams," through the use of beam forming antennas. Each beam covers a particular geographic region within a satellite's footprint.
Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters," and U.S. Pat. No. 5,691,174, which issued Nov. 25, 1997, entitled "Method and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy," both of which are assigned to the assignee of the present invention, and are incorporated herein by reference.
In communication systems employing CDMA, separate communication links are used to transmit communication signals to and from a gateway or base station in a cellular system. A forward communication link refers to communication signals originating at the gateway or base station and transmitted to a user terminal. A reverse communication link refers to communication signals originating at a user terminal and transmitted to the gateway or base station. In situations where satellite diversity is desired, the gateway establishes two or more forward links for a given user terminal, where each forward link is established on a beam from a different satellite. For example, in a two satellite diversity configuration a first forward link is established on a beam projected by a first satellite and a second forward link is established on a beam projected by a second satellite. In the above example, the user terminal receives information or data from the gateway on both the first and second beam. Satellite diversity provides increased system performance because fewer communication links or calls will likely be dropped. For example, if the beam carrying the first forward link is blocked by an obstruction (such as, a tall building), the connection between the user terminal and gateway will continue uninterrupted on the second forward link. The user will be unaware of the beam blockage. Consequently, beam source diversity is commonly desired in a multi-beam communication system.
In a satellite-based communications system where the satellites are not stationary with respect to a point on the surface of the earth, the geographic area covered by a given satellite is constantly changing. As a result, a user terminal that was at one time positioned within a particular beam of a particular satellite can at a later time be positioned within a different beam of the same satellite and/or within a different beam of a different satellite. Furthermore, because satellite communication is wireless, a user terminal is free to move about. Thus, even in systems where the satellites are stationary with respect to a point on the surface of the earth, it is likely that over time a user terminal will be covered by different beams. Consequently, if a communication link between a user terminal and a gateway is established on a first beam and the communication link is not established on other beams prior to the user terminal no longer being covered by the first beam, then, at some point, the user terminal will no longer be able to communicate with the gateway using the established communication link. As a result, an active call between the user terminal and the gateway will be dropped. Dropping calls in a communication system is a serious problem for service providers who strive to provide uninterrupted communication services. A similar call dropping problem may occur for mobile users moving around in sectored cells in terrestrial communication systems. That is, where the cells are subdivided into two or more smaller service areas which are covered at differing frequencies or using different code spaces. Here, mobile users may travel along or repeatedly cross sector boundaries within a cell, depending on such factors as cell and sector size and local physical environment.
What is, therefore, needed is a system and method for reducing call dropping rates in a multi-beam communication system. The system and method should maintain a desired level of beam source diversity to further enhance the reliability of the communication system.